Process for the preparation of printing plates



Patented on. 26, 1943 PROCESS FOR. THE PREPARATION OF PRINTING PLATESAdrian Eugene Le Roy and Jeannette Le Roy,

New York, N. Y., assignors, by memo assignments, to LectrocutCorporation Limited, a corporation of the Province of Ontario, Canada NoDrawing. Application May 6, 1939, Serial No. 272,140

1 Claim.

This invention relates to printing plates, particularly half-tone zincplates, including the process of making the same and the sensitizer andetching bath for same.

It is customary to make printing plates of zinc or copper by coating theplate with a sensitizer; impressing the image of the object to beprinted on the plate; washing the exposed plate; drying it; burning itin; and then placing. it in an acid bath for etching. The etchingprocess usually involves considerable time and care, including agitatingthe plate or etching fluid, stopping out with an acid resisting varnishthose parts which have been etched to the required extent, and makingsupplementary etchings when necessary.

The object of the present invention is to devise a process and means foretching the plate in a single quick operation requiring no specialskill.

The object of this invention is achieved by using a zinc plate andetching in an acid copper plating bath in which. the zinc plate is thecathode. More specifically the invention involves the useof a specialsensitizer, plating or etching bath, and electric current having a.distinctive characteristic, all of which are hereinafter moreparticularly described.

Ounces Water 96 Ammonium bichromate 3 Ferric ammonium citrate 1 Fish gl42 Ammonia water".- .5

This emulsion is prepared in the usual way and it is not necessary todescribe the manner of preparing the emulsion and applying it to theplate.

The developer and desensitizer for the plate are the same as in generaluse and need not be described.

when the photographic image has been formed on the plate and it has beenprocessed in the usual manner and is ready for etching it is placed inthe etching bath. The etching apparatus consists of, in effect, ordinaryelectro-plating apparatus, including a copper anode and a supportingbracket for the plate which is to be etched, the apparatus including asource of electrical current and being connected up in such a way thatthe plate comprises'the cathode.

The etching solution may be said in a general way to be the same as theordinary acid copper plating bath, with, however, a higher proportion ofacid. In this connection it should be pointed out that owing to theaction of acids on zinc, an acid bath is not usually used for platingzinc, and in this respect the present process diflers essen- V tiailyfrom an ordinary plating process.

Preferably also, there is added to the bath cobaltous chloride which hasthe effect of preventing pitting of the printing plate and a smallamount of ferric ammonium citrate for a purpose hereinafter descrlmd.

A typical bath contains the following constituents in the followingproportions:

Water (mixed at 140 2-1). 1 gal. or 3,785 mi.

Copper sulphate new... 3 lb. or 1,360.8 Cold water 3 gal. or 11,355 mi.Sulphuric acid 2% oz. or ml.

to which is added i. 02. or 29.6 ml. of the iollowing solution:

Cold water 20 oz. or 591.4 ml. Dye (Hercules red) Ii oz. or 29.6 ml.Cobaltous chloride 10 gr. or 0.648 gm. Ferric ammonium citrate 8 gr. or0.518 gm.

The following is a suitable replenishing solution for the bath:

When the plate has been suspended in the bath a current oi.substantially 6 volts 6 amperes is passed therethrough for substantiallysix minutes, for making a 5" x 7" plate, at the end of which time theetching will be complete and the plate may be removed, washed and driedand is ready for use. The amperage per square inch of cathode isappreciably higher than used in the ordinary copper electroplatingprocess.

The eflect of the Process is that the acid etches the zinc around thehalf-tone dot formed by the emulsion, and copper from the bath isdeposited on top 01' the dot, being attracted by the chromium content ofthe emulsion which forms the dot. In fact, a spongy copper depositcovers the whole plate, but does not interfere with the etching aroundthe dots and when the process is completed the soft copper may bereadily scrubbed off leaving a minute but readily observable harddeposit only on top of the dots formed by the sensitive emulsion.

The addition of the ferric ammonium citrate to the sensltizer forms aniron deposit which improves the plating effect, and the addition offerric ammonium citrate to the plating or etching solution forms acontact between the solution and the iron in the sensitizer. It isthought that the iron in the ferric ammonium citrate increases theconductivity of the sensltizer which forms the dot and of the solution,thus concentrating the plating eflect on top of the dot, whereas betweenthe dots the acid acts on the zinc to etch. it. The result is that thereis a simultaneous building up of the dots, or at least a hardening ofthe top of the dots to protect them from decomposition, and etchingaround the dots so that a clear cut image is etched in a comparativelybrief space of time. The process is remarkable for the uniformity of theetching, as well as the speed with which it is done, and the absence ofundercutting of the dots. This makes it unnecessary to remove and treatthe plate from time to time and makes it possible to obtain good resultswithout the exercise of the skill which is usually involved in ordinaryprocesses when it is necessary to agitate the bath and watch the platecarefully as the process proceeds and treat it and touch it up.

It should be noted that this process has nothing in common with theknown method of producing copper plates by deplating between the dots,in which method the printing plate is the anode.

What we claim as our invention is:

In the process of making a zinc half-tone printing plate, the steps ofimpressing an image through a half-tone screen on a zinc plate coatedwith a light sensitive emulsion to which has been added a smallproportion of ferric ammonium citrate; preparing the plate for etchingby developing, washing, drying, and burning it in; immersing theprepared plate in an acid copper plating bath in which the plate is thecathode, said acid copper plating bath consisting of a solutionconsisting or the following ingredients in substantially the followingproportions:

Water mm 4 Copper sulphate lbs 3 Sulphuric acid oz 24 ADRIAN EUGENE LEROY. JEANNE'I'I'E LE ROY.

